Optimization of ammonia dosing during regeneration

ABSTRACT

A method for managing ammonia dosing during a regeneration event for a nitrogen oxide particulate filter in (NPF) includes a selective catalytic reduction system and a particulate filter. Embodiments include utilizing monitored pressure changes across the NPF to predict the soot load and when the soot load will reach a level that triggers a regeneration event. The predicted timing of the regeneration event is used to utilize ammonia absorbed on the NPF and adjust the amount or rate of dosing ammonia to manage ammonia levels in the NPF during the regeneration event. Prior to the regeneration event, the amount of dosing ammonia may be reduced so that ammonia released from the NPF is utilized in converting nitrogen oxides to nitrogen gas and water. After the released ammonia has been utilized, the amount of dosing ammonia may be increased so that the conversion of the nitrogen oxides continues.

BACKGROUND

Combustion engines may employ emission controls or systems that are configured to reduce the amount of nitrogen oxides (NOx), such as nitrogen dioxide, present in the engine's exhaust gas. One aspect of controlling such emissions may include the use of a NOx particulate filter (NPF) that has a Selective Catalytic Reduction (SCR) system and a particulate filter. For example, for diesel engines, the NPF may include an SCR and a diesel particulate filter. The particulate filter is configured to remove particulate matter, such as soot, from the exhaust gas. The SCR typically uses a catalyst, which, in some designs, may be coated on the particulate filter, and a reductant to convert NOx in the exhaust gas into nitrogen gas and water. Typically, the reductant is injected into the exhaust gas before the exhaust gas enters the NPF. The reductant may be a liquid or gas, such as, for example, urea, anhydrous ammonia, or aqueous ammonia, among others. The reluctant may provide, or provide for the formation of, ammonia (NH₃) in dosing amounts or rates for the conversion of NOx in the SCR. The exhaust system may determine the appropriate amount or rate of dosing ammonia that is needed for the conversion of the NOx in the SCR. Ammonia besides the dosing ammonia may also be present in the NPF, such as, for example, ammonia that has been collected, absorbed, or stored on catalytic sites within the NPF.

The accumulation of particulate matter, such as soot, in the NPF may require that the NPF occasionally be cleaned or replaced. One approach to cleaning the NPF is to remove or reduce the amount of accumulated soot by a regeneration, or oxidation, process or event. During the regeneration event, the temperature within the NPF is increased, e.g., to around 550° C., to heat the soot to at least the combustion temperature of the soot. However, the high temperatures in the NPF during the regeneration event may cause ammonia that has collected on catalytic sites within the NPF to be desorbed or released in the NPF. Yet, while ammonia collected in the NPF may be desorbed or otherwise released from the NPF during the regeneration event (referred to herein as released ammonia), the exhaust system may continue introducing the dosing ammonia into the exhaust gas at the same rate or in the same amount as before the regeneration event. The inclusion of both the released ammonia and the dosing ammonia in the NPF may result in an overabundance of ammonia. As a result, excess ammonia that is not used to convert NOx in the SCR may be released into the exhaust stream and into the atmosphere. Further, the ability of the NPF to convert NOx to nitrogen gas and water, and more specifically the efficiency of the catalyst of the SCR, may diminish as the temperature within the NPF reaches and exceeds the temperatures needed for a regeneration event. Thus, a decrease in the efficiency of the catalyst may increase the quantity of unused ammonia in the NPF, and thus result in unused ammonia slipping through the NPF. For example, if the NPF is filled to a target capacity of dosing ammonia when the regeneration event starts, the released ammonia, as well as an amount of dosing ammonia that has not been utilized due to a decrease in the catalyst's efficiency, may slip through the NPF.

One attempt to prevent ammonia that has slipped through the NPF from being released into the atmosphere is to position a precious metal based ammonia slip catalyst downstream from the NPF. A precious metal ammonia slip catalyst may oxidize the ammonia to produce nitrogen gas. However, the inclusion of such a slip catalyst system and its precious metal adds to both the cost and the complexity of the emission system.

SUMMARY

Embodiments depicted herein related to a method for managing an ammonia dosing strategy for the conversion of NOx in a NPF that includes a selective catalytic reduction system and particulate filter for a combustion engine. In one embodiment, the method includes monitoring a change in pressure across at least a portion of the NPF. The method also includes predicting when a regeneration event within the NPF will be initiated based on the monitored pressure change across the NPF. Additionally, the method includes adjusting the amount of dosing ammonia provided to the NPF when the regeneration event is predicted to be initiated.

In another embodiment, the method includes monitoring a change in pressure across at least a portion of the NPF and determining a soot load level in the NPF using the monitored pressure changes across the NPF. The method further includes predicting when soot load level will reach a predetermine level that will trigger a regeneration event. Additionally, the amount of dosing ammonia provided to the NPF in advance of the regeneration event is reduced in order to prevent the accumulation of excess ammonia in the NPF during the regeneration event. Further, the amount of ammonia stored on the NPF is depleted in advance of the regeneration event being triggered by being used in the conversion of NOx in the selective catalytic reduction system.

According to a further embodiment, the method includes predicting when a regeneration event will be initiated within the NPF. The regeneration event is configured to combust soot that has accumulated in the NPF. Ammonia stored in the NPF is released in advance of the regeneration event. The released ammonia is used in the selective catalytic reduction system for the conversion of NOx. The amount of reductant injected into the exhaust gas entering the NPF is decreased while the released ammonia is used in the conversion of NOx in the selective reduction system. Further, the amount of reductant injected into the exhaust gas after the released ammonia has been at least partially depleted is increased so as to continue the conversion of NOx in the selective reduction system.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a function block diagram of an engine system including an exhaust gas treatment system.

FIG. 2 is a flow chart illustrating an exemplary method for optimizing ammonia dosing of exhaust gas during active regeneration.

DETAILED DESCRIPTION

FIG. 1 is a function block diagram of an engine system 10 that includes an exhaust gas treatment system 12. As shown, the engine system 10 includes a combustion engine 14, such as for example, and a diesel, gasoline, or petrol engine. The engine system 10 may also include an exhaust manifold 16 that couples the combustion engine 14 to the exhaust gas treatment system 12. The exhaust gas treatment system 12 may include one or more exhaust pipes 18 configured to transport engine exhaust gas along the exhaust gas treatment system 12 to, and out of, a tailpipe 20.

The exhaust gas treatment system 12 may also include an SCR 22, and particulate filter 24, and an engine control unit 26. As shown in FIG. 1, according to certain embodiments, the SCR 22 and particulate filter 24 may both be part of an NPF 27. As shown in FIG. 1, according to certain embodiments, the exhaust gas treatment system 12 may further include an exhaust gas temperature sensor 28, a first NOx sensor 30, and a flow sensor 32 that are positioned between the combustion engine 14 and the NPF 27. While these sensors 28, 30, 32 are illustrated in FIG. 1 as being upstream of the SCR 22, according to certain embodiments in which the SCR 22 is not coated on the particular filter of the NPF 27, these exhaust gas sensors 28, 30, 32 may be positioned downstream of the SCR 22 while remaining upstream of the NPF 27. Information indicative of the sensed temperature, NOx level, and/or flow rate of the exhaust gas before entry into the NPF 27 may be sent to the engine control unit 26 or other control unit or module used to monitor the conditions of the exhaust gas. Alternatively, rather than, or in addition to, using the NOx sensor 30, an emission controller, which may or may not be part of the engine control unit 26, may predict the NOx level, as discussed below.

According to certain embodiments, the NPF 27 includes one or more NPF pressure sensors 34 and a NPF temperature sensor 36. Information indicative of the pressure sensed by the NPF pressure sensor(s) 34 and the temperature sensor 36 are also sent to the engine control unit 26 or other control module. Detected pressure changes in or across the NPF 27 may be used, such as, for example, by the engine control unit 26, in predicting when the soot load in the NPF 27 will reach a level that will trigger a regeneration event.

The exhaust gas treatment system 12 may also include a second NOx sensor 38 that is positioned between the outlet of the NPF 27 and an exhaust outlet, such as the outlet of the tailpipe 20. As shown in FIG. 1, according to certain embodiments, the second NOx sensor 38 is positioned within the tailpipe 20. The second NOx sensor 38 provides information used by the engine control unit 26 or other control module in determining the amount of NOx present in exhaust gas downstream of the NPF 27. Such information may be used in a feedback ammonia injection strategy, as discussed below.

FIG. 2 is a flow chart illustrating an exemplary method 100 for optimizing ammonia dosing of exhaust gas during active regeneration. The exemplary method 100 includes step 110, in which the amount of NOx released after combustion of fuel or other materials is detected or predicted upstream of the NPF 27. For example, according to an embodiment, following the combustion of fuel, such as a diesel fuel, biodiesel fuel, gasoline, lean combustion, or fuel for other combustion systems, the amount of NOx in the exhaust gas at a location upstream of the NPF 27 may be detected by a NOx sensor 30. According to another embodiment, the NOx level of the exhaust gas upstream of the NPF 27 may be predicted by an emission controller that is operably connected to, or part of, the engine system, such as the engine control unit 26. Moreover, the emission controller may predict the NOx level based on a number of different factors and variables, including, for example, the type of fuel or material being combusted, intake air pressure and temperature, and/or the predicted or sensed combustion and/or exhaust temperature, among other factors.

At step 120, the information detected or predicted at step 110 is used in determining a catalyst model for converting NOx to nitrogen gas and water. The catalyst model is directed to predicting the amount of ammonia needed to convert the NOx to nitrogen gas and water based on the actual or predicted condition of the exhaust gas and the actual or predicted conditions inside the SCR 22. Moreover, the catalyst model may take into account the ratio of NO to NO₂ present in the exhaust gas, the temperature and flow rate of the exhaust gas, and the amount of ammonia stored in the NPF 27, among other factors. The catalytic model may also predict the amount of ammonia that will be absorbed or consumed during the conversion of the NOx in the SCR 22.

Information from the catalyst model is provided to both a feed forward ammonia injection strategy and a feedback ammonia injection strategy at steps 130, 140, respectively. The feedback injection strategy, shown at step 140, is also provided with information regarding the amount of NOx detected or sensed in the exhaust gas downstream of the NPF 27. More specifically, at step 150, one or more NOx sensors 38 positioned downstream of the NPF 27, such as, for example, the NOx sensor 38 positioned in the tailpipe 20, detects the amount of NOx present in the exhaust. The feedback ammonia strategy uses the information and/or conditions detected at step 150, as well as the catalyst model from step 120, to predict the amount dosing ammonia or reductant needed for the conversion of NOx in the SCR 22.

In addition to information provided by the catalytic model at step 120, the feed forward ammonia injection strategy at step 130 also receives information from both the feedback injection strategy of step 140 and a regeneration event evaluator, shown at step 160. The feed forward injection strategy predicts the amount of ammonia or reductant to inject into the exhaust gas based on these actual or predicted conditions. Although shown in the illustrated embodiment as separate steps, according to certain embodiments, the feed forward and feedback ammonia injection strategies may be unified into one step that receives information from at least both the catalytic model (step 120), the downstream NOX sensors 38 (step 150), and the regeneration event evaluator (step 160).

The regeneration event evaluator assists with actively managing ammonia storage within the NPF 27 so as to minimize wastage of ammonia during a regeneration event. For example, the regeneration event evaluator monitors and/or predicts when a regeneration event is imminent, or will be triggered, so that the amount and/or rate of dosing ammonia may be adjusted to reduce the likelihood of excess ammonia slipping through the NPF 27 during the regeneration event. According to an embodiment, the regeneration event evaluator may be a particulate filter regeneration control module that is located in a control unit of the engine system 10, such as, for example, the engine control unit 26. Generally, the engine control unit 26 may control various engine functions and operations, such as, for example, controlling an electronic fuel injection system, among other tasks.

A number of different triggers may result in the initiation of a regeneration event, including, for example, the expiration of a time period since the previous regeneration event, the actual or predicted soot load in the NPF 27 reaching a predetermined or maximum level, and/or gas flow rates through the NPF 27, among other triggers. According to an embodiment, one or more pressure sensors 34 are position along the NPF 27 to continuously or periodically measure changes in pressure across the NPF 27. Such sensed or measured pressure changes may be used by subsequent logic, calculations, and/or an algorithm(s) to predict the soot load in the NPF 27 and to predict when the soot load will reach a level that will trigger a regeneration event. When a predicted regeneration event is going to be triggered or occur can be presented in different units, such the time of day, time of operation, engine strokes, and/or miles (for embodiments used with vehicles), among others. For example, according to an embodiment, sensed or measured changes in pressure may be mapped and/or calibrated to a known soot level by the particulate filter regeneration control module or other control module to predict the number of miles before the regeneration event will, or is predicted to, be triggered.

Information from the regeneration event evaluator regarding when the next regeneration event will be, or is predicted to be, triggered is provided to the feed forward ammonia injection strategy (step 130). The feed forward ammonia injection strategy uses this information to determine a dosing ammonia strategy that eliminates or reduces the potential for excess ammonia in the NPF 27 when the regeneration event occurs. The feed forward ammonia injection strategy will take into account that during operation of the combustion engine 14, NOx is continually present in exhaust gas from the combustion engine 14 following combustion of fuel. Thus, according to an embodiment, by predicting when a regeneration event is, or is predicted, to be triggered, and the predicted amount of released ammonia that will be present in the NPF 27 during the regeneration event, the rate or amount of dosing ammonia may be adjusted, e.g., reduced, to prevent the presence of excess (unused) ammonia in the SCR 22 during the regeneration event due to the presence of both the released and dosing ammonia.

According to another embodiment, the ammonia dosing strategy may involve reducing or eliminating the amount or rate of dosing ammonia prior to the regeneration event so that ammonia present or stored in the NPF 27 is utilized in advance, or before, the regeneration event. Following the depletion or decrease in the amount of the released ammonia in the NPF 27, the amount or rate of dosing ammonia during the regeneration event may then be optimized in view of the elevated regeneration temperatures so as to prevent or minimize the amount of ammonia that slips through the NPF 27 while still providing sufficient ammonia for NOx conversion in the SCR 22.

According to certain embodiments, to assist in utilizing the ammonia in the NPF 27 before the regeneration event, the temperature within the NPF 27 may be elevated to release ammonia stored in the NPF 27. According to such an embodiment, the amount or rate of dosing ammonia may be reduced to compensate for the addition of released ammonia in the NPF 27. After the released ammonia in the NPF 27 has been depleted or at least partially depleted, the amount or rate of the dosing ammonia may be adjusted to accordingly to continue the conversion of NOx in the SCR 22 during the regeneration event while seeking to prevent ammonia from slipping through the NPF 27.

Besides, or in addition to, providing information to the feed forward ammonia injection strategy, the regeneration event evaluator may also provide information as to the predicted occurrence of the regeneration event to other control modules or portions of the engine control unit 26 to further control NOx emissions during a regeneration event. For example, the timing of the regeneration event, as predicted by the regeneration event evaluator, may be used by the engine control unit 26 to manipulate engine operations, such as, for example, the fuel injection system, among others, so as to reduce the amount of exhaust gas being produced, and thus reduce the amount of NOx being produced, at or around the expected occurrence of the regeneration event. Reducing the amount of NOx upstream may allow for reducing the amount or rate of dosing ammonia needed for the NOx conversion in the SCR 22, and thereby reduce the amount of ammonia that may slip through the NPF 27 during the regeneration event.

At step 170, the feed forwarded and feedback ammonia injection strategies of steps 130, 140 are compiled to create an actual ammonia dosing rate, shown at step 180. The compiled dosing strategies therefore not only take into account the amount of NOx present in the exhaust gas upstream and downstream of the NPF 27, but also manage the amount of ammonia present in the NPF 27 in advance of the regeneration event and/or when the regeneration event is triggered. 

1. A method for managing an ammonia dosing strategy for the conversion of nitrogen oxides in a nitrogen oxide particulate filter that includes a selective catalytic reduction system and particulate filter for a combustion engine, the method comprising: monitoring a change in pressure across at least a portion of the nitrogen oxide particulate filter; predicting when a regeneration event within the nitrogen oxide particulate filter will be initiated based on the monitored change in pressure across the nitrogen oxide particulate filter; and adjusting the amount of dosing ammonia provided to the nitrogen oxide particulate filter when the regeneration event is predicted to be initiated.
 2. The method of claim 1, further comprising determining a soot load within the nitrogen oxide particulate filter based on the monitored changed in pressure.
 3. The method of claim 2, wherein the step of determining when the regeneration event will be initiated includes determining when the soot load in the nitrogen oxide particulate filter will reach a predetermined level.
 4. The method of claim 1, further comprising manipulating operation of the combustion engine to reduce the amount of nitrogen oxide produced by the combustion engine during the regeneration event.
 5. The method of claim 1, wherein the step of adjusting the amount of dosing ammonia includes reducing the amount of dosing ammonia present in the nitrogen oxide particulate filter in advance of the predicted regeneration event so that ammonia released from the nitrogen oxide particulate filter is utilized for the conversion of nitrogen oxides in the selective catalytic reduction system.
 6. The method of claim 5, wherein the step of adjusting the amount of dosing ammonia further includes increasing the amount of dosing ammonia present in the nitrogen oxide particulate filter after the released ammonia that was stored in the nitrogen oxide particulate filter has been at least partially depleted through the conversion of nitrogen oxides in the selective catalytic reduction system.
 7. A method for managing an ammonia dosing strategy for the conversion of nitrogen oxides in a nitrogen oxide particulate filter that includes a selective catalytic reduction system and particulate filter for a combustion engine, the method comprising: monitoring a change in pressure across at least a portion of the nitrogen oxide particulate filter; predicting a soot load level in the nitrogen oxide particulate filter using the monitored change in pressure across the nitrogen oxide particulate filter; predicting when the predicted soot load level will reach a predetermine level that will trigger a regeneration event; reducing the amount of dosing ammonia provided to the nitrogen oxide particulate filter in advance of the regeneration event to prevent the accumulation of excess ammonia in the nitrogen oxide particulate filter during the regeneration event; and releasing ammonia stored on the nitrogen oxide particulate filter, the released ammonia being used in the conversion of the nitrogen oxides in the selective catalytic reduction system.
 8. The method claim of claim 7, further including increasing the amount of dosing ammonia after the released ammonia has been depleted by the conversion of nitrogen oxides in the selective catalytic reduction system.
 9. The method of claim 8, wherein the amount of dosing ammonia is increased in advance of the regeneration event.
 10. The method of claim 8, wherein the amount of dosing ammonia is increased during the regeneration event.
 11. The method of claim 7, further comprising manipulating operation of the combustion engine to reduce the amount of nitrogen oxide produced by the combustion engine during the regeneration event.
 12. A method for managing an ammonia dosing strategy for the conversion of nitrogen oxides in a nitrogen oxide particulate filter that includes a selective catalytic reduction system and particulate filter for a combustion engine, the method comprising: predicting when a regeneration event within the nitrogen oxide particulate filter will be initiated, the regeneration event configured to combust soot that has accumulated in the nitrogen oxide particulate filter; releasing ammonia stored in the nitrogen oxide particulate filter in advance of the regeneration event, the released ammonia being used in the selective catalytic reduction system for the conversion of nitrogen oxides; decreasing the amount of reductant injected into the exhaust gas entering the nitrogen oxide particulate filter while the released ammonia is being used in the conversion of nitrogen oxides in the selective reduction system; and increasing the amount of reductant injected into the exhaust gas after the released ammonia has been at least partially depleted to continue the conversion of nitrogen oxides in the selective reduction system.
 13. The method of claim 12, wherein the step of predicting when the regeneration event will be initiation includes monitoring changes in pressure across the nitrogen oxide particulate filter.
 14. The method of claim 13, wherein the monitored pressure changes and predicted soot load are used to predict when the soot level will reach a predetermined triggering level.
 15. The method of claim 12, further comprising manipulating operation of the combustion engine to reduce the amount of nitrogen oxide produced by the combustion engine during the regeneration event. 